chore(github): Replace feature request as issues with ideas as discussions

[Savio-Sou]

Description

Problem*

Noir's feature requests are currently collected and managed as GitHub Issues.

This is not ideal as:

Suggesting "Issues" is unintuitive

The term "Issue" could carry the meaning of "problems / bugs", which leads to GitHub Issues not necessarily being the first place users would check if they would like to raise a suggestion.

Upvoting is unintuitive

The thumbs-up emoji can be used as a pseudo upvote, but it is hidden behind the "reaction" smiley face and doesn't show up in the Issue list unless explicitly sorting with it.

Mental barrier

Filing a GitHub Issue often comes with a mental barrier that it should be logically-detailed and well-written, which might deter users from creating a suggestion.

Summary*

GitHub Discussions could potentially be a better place for collecting and managing feature requests.

This PR:

• Removes the existing feature request GitHub Issue template (in favor of the org-wide ideas GitHub Discussions template)
• Adds an "Ideas" contact link in the list of actions on the "New Issue" page (for users that are used to / prefer creating feature requests through the creating GitHub Issues workflow)

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.

Additional Context

For a glimpse of how this would look like in action, checkout the demo noir-lang org: https://github.com/noir-lang-test/noir/issues/new/choose

For ideas that contributors would like to work on, they can easily convert discussions into GitHub Issues and follow up with the usual GitHub engineering workflow:

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[TomAFrench]

This is a bit of a pain as I can no longer create feature requests issues directly and get pushed towards spamming the discussions page or have to make do without the template.

[Savio-Sou]

Right, it's a choice of tradeoffs between hosting Feature Requests as:

1. Issues (previous practice)
   + Can create executable pieces of work directly
   - Unintuitive / deterring for the community

2. Discussions first, then convert to Issues when executing (as per this PR)
   + Eases community engagement
   + Clear boundary between what is welcoming votes & pending for contribution (Discussions) <> what is already in execution (Issues)
   - 2 additional clicks to create executable pieces of work

3. Both (1) and (2)
   + Can create executable pieces of work directly
   - Potentially even more confusing than (1):

   • Multiple paths to request feature through "New Issue" (as an Issue or Discussion)
   • Vague status of open Issues: are they welcoming votes or already in execution
   • Mixed voting signals for prioritising works (e.g. are 3 upvotes on Discussion equivalent to 3 thumbs-up on Issue?)

Certainly jumped the gun and assumed (2) gives us the best set of tradeoffs. Would you consider a switch back to (1) is justifiable by the added friction?

Definitely open to adopting an option (4) that achieves the best of both worlds too, suggestions welcome!